With increased computerization, more and more sensitive and valuable information is being generated and stored. The need for storage capacity is increasing. Higher capacity backup tapes are demanded.
One method for storing information on magnetic tape uses what is a known as "helical scan" technology. Helical scan tape systems cause information to be recorded in stripes that are diagonal relative to the length of a tape. The helical scan systems use a rotating drum head operating on a slowly driven tape, for high capacity. The tape is wrapped around the rotating drum.
Another method for storing information on magnetic tape uses what is known as "linear recording" technology. Linear recording tape systems cause information to be recorded in multiple parallel tracks that extend in the direction of the length of the tape. Linear recording systems use a stationary head operating on tape that is driven past the head at a speed that is typically much faster than the speed used by helical scan tape systems. With linear recording systems, multiple read/write elements can be employed in a head and can simultaneously operate on the tape.
Continuing advances in magnetic storage technology provide for constantly increasing data storage densities. The more tracks that can be fit across the width of a tape, the more information that can be stored on a tape. Thus, there is a push to increase the number of tracks across the width of a tape and to reduce the width of each track. Increasing the number of tracks or reducing the width of a track makes accurate positioning of read/write elements, and issues of tape wander more important. Therefore, servo systems are desirable in linear drive systems.
Servo systems employ information or patterns recorded along a track of the tape to accurately position read/write elements relative to data on the tape. The servo information can be used to accurately position heads relative to the length of the tape (e.g., when searching for a desired position along the length of the tape, such as the start of a file) as well as relative to the width of the tape.
Thus, servo patterns on a tape have a characteristic that changes across the width of the tape. Dual frequency servo schemes are known, in which a pattern is written with one frequency on one side of the servo track, and at a different frequency at the other side of the track. Thus, as a servo element passes over the servo track, lateral position can be determined.
Various servo systems are known in the art. For example, U.S. Pat. No. 5,432,652 (incorporated herein by reference) relates to a magnetic tape that has three evenly spaced-apart longitudinally-extending servo track areas. Four equal-sized longitudinally-extending data track areas are disposed between the servo track areas and between longitudinal edges of the tape and one of said longitudinally-extending data track areas. For track following, all servo track areas are simultaneously sensed for producing one head positioning signal.
U.S. Pat. No. 5,008,765 (incorporated herein by reference) relates to a method for reading or writing data on a tape which has a plurality of data tracks and at least a first dedicated servo track. A multiple channel head is used to access the tracks on the tape. The head is moved proximate one of a plurality of predetermined positions. The channels are located so that, in any one predetermined position of the head, one channel accesses the center of a dedicated servo track on the tape and at least two other channels will access the center of distinct data tracks.
U.S. Pat. No. 5,262,908 (incorporated herein by reference) relates to a tracking control device for a magnetic recording/reproducing apparatus arranged in such a manner that a head unit having a plurality of magnetic heads is successively moved in the widthwise direction of a magnetic tape for switching racking positions so that data recording/reproducing is, by each of the plurality of magnetic heads, performed along a plurality of data tracks formed on the magnetic tape in parallel to a direction in which the magnetic tape moves.
U.S. Pat. No. 5,574,602 (incorporated herein by reference) relates to a magnetic tape drive. A magnetic head simultaneously senses plural track lateral position indicators to generate a like plurality of independently generated sensed position error signals. The sensed position error signals are combined to provide an output position error signal that drives a positioning system to position the magnetic head laterally of the length of the magnetic tape. The output position error signal represents an average of the position errors indicated by the sensed position error signals. The quality of the sensed position error signal is monitored, eliminating poor quality signals from the output position error signal for maintaining a quality servo control.
U.S. Pat. No. 5,450,257 (incorporated herein by reference) relates to a head-track orienting system for use in magnetic recording tape drives which automatically corrects for misalignment between the head assembly and a recorded servo track on the tape. Using a servo control loop, the system calculates head-track alignment error during operation of the tape drive and either pivots the head assembly or adjusts the tape cartridge to compensate for the error. Transverse head-track positioning mechanisms are also included in the system to locate and maintain a centered position of the heads on the servo track.
It is desirable for a servo system to be able to tolerate defects in the magnetic layer of the media. For example, some tape drive systems can tolerate long scratches in a data track. So called "infinite" scratches do not cause stream failures in a data channel generated by the data track. The data tracks are recovered using error control code techniques known in the art. It is desirable to have servo systems that can tolerates defects in the servo track as capably as the defects in the data track are tolerated. It is therefore desirable to have servo systems that can tolerate long scratches in a servo track and scratches or other defects extending in the direction of the width of the tape.